High purity fused silica glass has found use in optics of many devices due to its excellent performances and properties, such as optical transmission, thermal expansion, and the like. Particularly, in the area of state-of-the-art deep UV and vacuum UV projection lithography, high purity synthetic silica glass has become the material of choice because other glass materials otherwise useable at longer wavelength are disqualified.
The desire of higher resolution in microlithography in the semiconductor chip industry has placed more stringent requirements on the properties and performances of the materials used in the optics in manipulating the lithographic irradiations. Such critical properties for the silica glasses used in projection lithography used in ArF excimer laser microlithography include, but are not limited to, static properties such as initial internal transmission, refractive index homogeneity, initial birefringence, fictive temperature, and the like, but also dynamic properties in response to laser exposure such as light induced wavefront distortion (LIWFD), light induced birefringence, fluence-dependent transmission (FDT), induced absorption (IA), and the like.
Recently, with the advent of immersion lithography, in which polarized UV irradiation is used, research results on behavior of silica glass material when exposed to polarized light have been published. It was found that for certain silica glasses, for reasons not well understood, tends to suffer from a problem we call polarization-induced birefringence (PIB). Particularly, if the glass is exposed to linearly polarized light, birefringence is produced in the exposed area. When the material develops PIB there is undesirable modification of the incident exposing light, leading to image distortion.
Therefore, there is a genuine need of a silica glass material suitable for lithography applications operating at below about 300 nm, such as at about 248 and 193 nm, which demonstrates lower polarization-induced birefringence than existing materials, or better yet, essentially no polarization-induced birefringence.